The weft thread supply in modern rapidly operating air nozzle weaving looms is accomplished by so-called weft prewinders which provide weft thread sections of predetermined length in accordance with the weft insertion capacity of the loom. The released weft section length depends on the width of the fabric being woven. A released weft thread section is stopped at its tail end under the control or rather in response to the loom control that controls the weft insertion into the loom shed. Prior to the sequential release of weft thread sections, the weft thread is wound on the weft prewinder. The weft controller is arranged downstream of the prewinder, as viewed in the weft advance direction. The weft controller is a weft stopper functioning as a clamp controllable through an electromagnet to either stop or release the weft thread.
In the course of a weft thread insertion the loom control electrically controls the main weft insertion nozzle and the prewinder in synchronism with each other in such a way that the weft controller or stopper of the weft prewinder and the valve of the main nozzle are operated in synchronism for the release of the required weft length to be inserted into the loom shed by the nozzle. This synchronous control includes the electrical energizing of the electromagnetic control valve of the main weft insertion nozzle. The same synchronous control activates the weft stopper at the end of a weft thread insertion sequence. Thus, it is assured that the weft length released by the weft prewinder corresponds to the required weft thread insertion length. For this purpose the diameter of the prewinder drum is adjustable and the prewinder drum releases a whole number of turns the total length of which corresponds to the required weft thread insertion length for the fabric being woven. For these functions conventional weft prewinders are equipped with at least one weft stopper or weft controller which is radially positionable relative to the prewinder drum diameter for sequentially releasing the leading end of a weft thread or stopping the trailing end of the weft thread.
European Patent EP 0,544,730 (Josefsson et al.) discloses a method for controlling a weft thread supply and measuring device. Such a device includes at least one weft thread stop that is activated either directly by an electromagnet or indirectly in response to the switching of an electromagnetically operated valve of the main nozzle. As disclosed in the just mentioned European Patent, the synchronous operation of the electromagnetic stopper of the prewinder and the magnetic valve of the main nozzle of an air nozzle weaving loom requires as a precondition that the point of time when a weft thread is released, the point of time when a released weft thread must be stopped, and the operational times of the main nozzle must properly coincide with each other or these times must at least be within certain acceptable tolerance limits. The prior art does not monitor, whether these preconditions for the synchronous operation of the loom continue to exist once weaving has started and continues.
When the weft controller or stopper of the prewinder releases the stopped weft thread with a delay, for example the stopping phase is too long by a few milliseconds after the begin of the weft insertion by the main nozzle, the weft thread between the prewinder and the main nozzle is exposed to a relatively high tension force. Similarly, when the stopper releases the weft thread too early, proper weft tensioning may not be possible. The relatively high tension force caused by a delayed stopper release can lead to weft thread breaks, depending on the yarn characteristics, such as the tensile strength of the weft thread. The break may occur directly during the just mentioned relative short time delay or it may occur during the further complete insertion of the weft thread into the loom shed.
Without the proper monitoring of the points of time required for the above mentioned synchronization between the stopper function and the main weft insertion nozzle it is conventionally not possible to directly recognize the cause for a weft thread break.
Similarly, stopping the weft thread too late, namely at a point of time after the rated or control stopping point of time, also leads to production faults because now the weft thread section is too long and hence not properly tensioned. The weft section is tool long because the prewinder dispensed an additional length of weft thread corresponding to a full turn of the weft thread prewinder. Such additional length is unnecessary for the current weft insertion. Under this condition of delayed stopping, as opposed to delayed release, weft loops can be formed that appear as faults in the fabric, or that can also lead to weft thread breaks.
The above mentioned U.S. Pat. No. 5,787,937 (Teufel), relates to a method for monitoring the proper functioning of electromagnetic air valves by evaluating a characteristic dip or drop in the energizing current characteristic of the electromagnet that drives the valve which provides air for the main weft insertion nozzle. Such monitoring provides an early recognition of possible defects or failure in the electrically controllable electromagnet that operates the valve of the main weft insertion nozzle in a loom. The disclosure of the above U.S. Patent recognizes the fact that all electromagnetic systems including electromagnetically operated valves have a characteristic curve of their drive current I as a function of time t(I=f(t)). This drive or actuating current characteristic has a short duration current dip or current reduction after the switch-on point of time. This dip in the energizing current occurs when the driven magnetic valve actually, physically switches over following switching on of the power for energizing the valve drive electromagnet. The prior art does not recognize the general applicability of the above actuating current characteristic for control and synchronizing purposes.